In the chloro alkali process, which produces caustic soda and chlorine, an ion-exchange membrane process is widely adopted, and a laminated type of membrane comprising a perfluorosulfonic acid polymer and a perfluorocarboxylic acid polymer is usually used as an ion-exchange membrane, which is a diaphragm for the process. In general, the perfluorocarboxylic acid polymer having a structure represented by the following general formula (5): (wherein k/l=3–15, m=0–1, n′=1–5, each of k, l, m and n′ is an integer, respectively),are used therein.Among them, those with n′=2–3 are usually used. This polymer can be obtained by the hydrolysis reaction of the film of the copolymer of perfluorovinylcarboxylic acid ester represented by the following general formula (6): (wherein m=0–1, n′=1–5, each of m and n′ respectively represent an integer, and R″ represents an alkyl group having 1–4 carbon atoms) and tetrafluoroethylene (TFE).
Heretofore, various methods have been proposed as a method for producing perfluorovinylcarboxylic acid ester. For example, Japanese laid-open publication No. 52-78827 discloses a method for producing perfluorovinylcarboxylic acid ester represented by the following general formula (2):CF2═CFO(CF2)nCO2R  (2)(wherein n represents an integer of 2–3, and R represents an alkyl group having 1–4 carbon atoms (i.e. m=0 and n′=2–3 in the above described general formula (6))),wherein a compound represented by the general formula (4) mentioned below, a starting material, is saponified in the acid fluoride moiety thereof to obtain its alkaline metal salt, and the alkaline metal salt is further subjected to a thermal decomposition reaction to obtain perfluorovinylcarboxylic acid ester. In this process, however, it is difficult to obtain perfluorovinylcarboxylic acid ester in a satisfying yield because various kinds of byproducts are produced.
For avoiding such side reactions, for example, a method wherein a vinyl group is formed through a dehalogenation reaction of a precursor having a ICF2CF2O— structure (Japanese laid-open publication No. 55-31004), and a method wherein a vinylether compound having a terminal CH3OCF2CF2— group is treated with a strong acid to introduce an ester group (Japanese laid-open publication No. 60-156632) have been proposed. These methods, however, have various problems in that it is needed, for example, to use expensive raw materials requiring multiple steps.
Japanese publication of examined application No. 45-22327 and Journal of Organic Chemistry 34, 1841(1969) disclose a method for obtaining perfluorovinylcarboxylic acid ester (a compound with n=2–12 in the general formula (2)), wherein a perfluorodicarboxylic acid salt (a compound with n=2–12 in the general formula (3) mentioned below) is thermally decomposed at 175–200° C. in the absence of solvent under anhydrous conditions to obtain perfluorovinylcarboxylic acid salt (a compound with n=2–12 in the following formula (1)), and subsequently the perfluorovinylcarboxylic acid salt is converted into a perfluorovinylcarboxylic acid by treating with an acid followed by reacting with an alcohol to obtain a perfluorovinylcarboxylic acid ester. In the method described in the publication of examined application, the manner to obtain the perfluorovinylcarboxylic acid ester from the perfluorovinylcarboxylic acid salt needs 2 steps of 1) producing the perfluorovinycarboxylic acid from the perfluorovinylcarboxylic acid salt by the acid treatment, and 2) producing the perfluorovinylcarboxylic acid ester by reacting the perfluorovinylcarboxylic acid with the alcohol. In addition, yield of perfluorovinylcarboxylic acid ester is very low. Further, the method has another drawback in that it requires a complicated purifying and separating process such as washing with a large volume of water to separate the perfluorovinylcarboxylic acid ester from the alcohol.
The inventors, after an extended study to solve the above described problems, found out a method to obtain a high-purity perfluorovinylcarboxylic acid ester in good yield, through less steps, by reacting perfluorovinylcarboxylic acid salt represented by the general formula (1) with an alkylating agent, and accomplished the present invention.